1. Field of the Invention
This invention relates to a magnetic toner for developing electrostatic latent images in image forming processes such as electrophotography and electrostatic printing. It also relates to a process for producing such a magnetic toner, and an image forming method making use of the magnetic toner.
2. Related Background Art
A number of methods are conventionally known as electrophotography, as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publication Nos. 42-23910 (U.S. Pat. No. 3,666,363) and 43-24748 (U.S. Pat. No. 4,071,361) and so forth. In general, copies or prints are obtained by forming an electrostatic latent image on a photosensitive member by utilizing a photoconductive material and by various means, subsequently developing the latent image by the use of a toner to form a toner image, and transferring the toner image to a transfer-receiving medium such as paper if necessary, followed by fixing by the action of heat, pressure or heat-and-pressure. The toner not transferred and remaining on the photosensitive member is removed by a various method to cleaning the photosensitive member surface, and the above steps are repeated.
As developing methods by which electrostatic latent images are developed by the use of toners, the magnetic brush development as disclosed in U.S. Pat. No. 2,874,063, the cascade development as disclosed in U.S. Pat. No. 2,618,552, the powder cloud development as disclosed in U.S. Pat. No. 2,221,776, the fur brush development and the liquid development are known in the art. In these developing methods, the magnetic brush development, the cascade development and the liquid development, which employ developers mainly composed of a toner and a carrier, are put into practical use. These methods are all superior methods which can relatively stably obtain good images, but on the other hand have problems inherent in the two component type developers, such that the carrier may: deteriorate and the mixing ratio between the toner and the carrier may change.
To eliminate such problems, developing methods employing one component type developers comprised of a toner only are proposed in variety. In particular, methods employing one component type developers having toner particles having magnetic properties are available.
U.S. Pat. No. 3,909,258 discloses a developing method employing a magnetic toner having an electrical conductivity. This is a method in which a conductive magnetic toner is supported on a cylindrical conductive sleeve having a magnet in its inside and the toner is brought into contact with electrostatic latent images to carry out development. In this development, in the developing zone, a conducting path is formed between the surface of an electrostatic latent image bearing member and the surface of the sleeve by conductive magnetic toner particles, and electric charges are brought from the sleeve to the conductive magnetic toner particles through the conducting path, where the conductive magnetic toner particles adhere to image areas of electrostatic latent images by the Coulomb force acting between the conductive magnetic toner particles and the images areas. Thus the electrostatic latent images are developed. This development carried out using a conductive magnetic toner is a method having avoided the problems inherent in the conventional two component type development. On the other hand, since the magnetic toner is conductive, there is the problem that it is difficult to electrostatically transfer the developed images from the electrostatic latent image bearing member to a final transfer-receiving medium such as plain paper.
As a developing method employing a high-resistivity magnetic toner that enables electrostatic transfer, there is a developing method utilizing dielectric polarization of magnetic toner particles. Such a method, however, has the problems that the development speed is substantially low and the density of developed images is not well attained.
As other developing methods employing a high-resistivity, insulating magnetic toner, methods are known in which magnetic toner particles are triboelectrically charged by the mutual friction between magnetic toner particles or by the friction between magnetic toner particles and a developing sleeve or the like and electrostatic latent images are developed using the magnetic toner having triboelectric charges. Such methods, however, have the problems that the triboelectric charging tends to be insufficient because of a small number of contact times between the magnetic toner particles and the friction member and the magnetic toner particles charged tend to agglomerate on the sleeve because of the Coulomb force increasing between the magnetic toner particles and the sleeve.
Japanese Patent Application Laid-Open No. 55-18656 (corresponding to U.S. Pat. Nos. 4,395,476 and 4,473,627) discloses novel jumping development that has eliminated the above problems. This is a method in which a magnetic toner is very thinly coated on a developing sleeve, and the toner thus coated is triboelectrically charged. Subsequently, the magnetic toner layer thus formed on the developing sleeve is made close to electrostatic latent images to develop the electrostatic latent images. According to this method, since the magnetic toner is very thinly coated on the developing sleeve, the opportunities of contact between the developing sleeve and the magnetic toner increase to enable sufficient triboelectric charging, and also since the magnetic toner is supported by magnetic force and the magnet and the magnetic toner are relatively moved, the magnetic toner particles are released from their mutual agglomeration and can be sufficiently brought into friction with the sleeve, whereby good images can be obtained.
In the insulating magnetic toner particles used in the above developing method, a finely divided magnetic material is mixed and dispersed in a considerable quantity and the magnetic material is partly laid bare to the surfaces of magnetic toner particles, and hence the properties of the magnetic material affect the fluidity and triboelectric chargeability of the magnetic toner, to consequently affect various performances such as developing performance and running performance required in magnetic toners.
Stated in detail, in the jumping development making use of a magnetic toner containing a conventional magnetic material, as a result of repetition of a developing step (e.g., copying) over a long period of time, the fluidity of the developer containing the magnetic toner may lower to make it impossible to achieve sufficient triboelectric charging, so that the charging tends to become non-uniform, and fog tends to occur in an environment of low temperature and low humidity, tending to cause problems on images. In the case when the binder resin and magnetic material that constitute magnetic toner particles have a weak adhesion, the magnetic material may come off the surfaces of magnetic toner particles as a result of the repetition of the developing step, so that a decrease in density of the toner images may occur.
In the case when the magnetic material is not uniformly dispersed in the magnetic toner particles, the magnetic toner particles containing the magnetic material in a large quantity and having small particle. diameters may accumulate on the developing sleeve to sometimes cause a decrease in image density and an uneven light and shade called sleeve ghost.
In the past, with regard to magnetic iron oxides contained in magnetic toners, Japanese Patent Application Laid-Open Nos. 62-279352 and 62-278131 (corresponding to U.S. Pat. Nos. 4,820,603 and 4,975,214, respectively) disclose a magnetic toner containing magnetic iron oxide particles incorporated with silicon element. In such magnetic iron oxide particles, the silicon element is intentionally brought into presence inside the magnetic iron oxide particles, but there is room for further improvement in the fluidity of the magnetic toner containing the magnetic iron oxide particles.
Japanese Patent Publication No. 3-9045 (corresponding to European Pat. Publication EP-A187434) discloses adding a silicate to control the shape of magnetic iron oxide particles to make spherical. In the magnetic iron oxide particles thereby obtained, the silicon element is distributed in a large quantity inside the magnetic iron oxide particles because of the use of the silicate for the controlling of particle diameter and the silicon element is less present on the surfaces of the magnetic iron oxide particles, so that the improvement in fluidity of the magnetic toner tends to become insufficient.
Japanese Patent Application Laid-Open No. 61-34070 discloses a process for producing triiron tetraoxide by adding a hydroxosilicate solution to triiron tetraoxide in the course of oxidation reaction for triiron tetraoxide. The triiron tetraoxide particles obtained by this process has silicon element in the vicinity of their surfaces, but the silicon element is present in layer in the vicinity of the surfaces of the triiron tetraoxide particles. Hence, there is the problem that the triiron tetraoxide particle surfaces are weak to mechanical shock such as friction.
To solve the above problems, the present inventors have proposed in Japanese Patent Application Laid-Open No. 5-72801 (corresponding to European Patent Publication EP-A533069) a magnetic toner containing magnetic iron oxide particles incorporated with silicon element and in which 44 to 84% of silicon element of the whole silicon content is present in the vicinity of the particle surfaces of the magnetic material.
The magnetic toner containing such magnetic iron oxide particles has brought about satisfactory improvements in the fluidity of magnetic toner and the adhesion between binder resin and magnetic iron oxide particles. However, because of the presence of silicic acid component in a large quantity on the outermost surfaces of the magnetic iron oxide particles disclosed, in Production Examples and because of the formation of porous structure at the surfaces of the magnetic iron oxide particles, the magnetic iron oxide particles have a large BET specific surface area. Hence the magnetic toner containing such magnetic iron oxide particles tends to cause a considerable lowering of triboelectric charging performance after it has been left in an environment of high humidity for a long period of time.
Japanese Patent Application Laid-Open No. 4-362954 (corresponding to European Patent Publication EP-A468525) also discloses magnetic iron oxide particles containing both silicon element and aluminum element, which, however, are sought to be more improved in environmental properties.
Japanese Patent Application Laid-Open No. 5-213620 still also discloses magnetic iron oxide particles containing a silicon component and in which the silicon component is laid bare to the particle surfaces, which, however, like the foregoing, are sought to be more improved in environmental properties.
Meanwhile, it is known in variety to previously coat-treating the particle surfaces of magnetic powder with an organic compound. For example, Japanese Patent Applications Laid-Open Nos. 54-122129 and 55-28019 disclose a method in which particle surfaces are coat-treated in an organic solvent using a silane compound (the former) or a titanium coupling agent (the latter). Such a method, however, may cause stiff agglomerates in the resultant magnetic powder when the organic solvent is removed. Hence, it becomes difficult to uniformly disperse the powder in the toner composition, causing faulty charging of toner and a phenomenon of come-off of powder from toner particles. Also, in the above method, the treating agent reacts in the reaction solution in so low an efficiency that the methods have poor economical advantages and also any unreacted treating agent having not participated in the particle surface coating of the magnetic powder may localize on the particles to cause a difficulty in its matching for image forming methods.
Japanese Patent Application Laid-Open No. 3-221965 also discloses a method in which the particle surfaces of a magnetic powder is treated with a treating agent such as a coupling agent by means of a wheel type kneading machine. According to this method, magnetic powder particles can be uniformly coated without causing any agglomerates but the rate of anchoring of treating agent on particles may lower. An attempt to increase OH groups on the particle surfaces of the magnetic powder or to enhance treatment strength in order to improve the anchoring rate may cause a decrease in the content of FeO in the magnetic powder, resulting in a decrease in blackness.
As a countermeasure to the above, Japanese Patent Application Laid-Open No. 6-230604 discloses a method in which oxide particles previously treated to make hydrophobic are anchored to the particle surfaces of the magnetic powder by a similar method. This method, however, because of a weak anchoring power of the oxide particles, requires a restriction of kneading strength when toners are produced, or has a problem on the running performance of toner.
Japanese Patent Application Laid-Open No. 63-250660 also disclose a magnetic toner produced by suspension polymerization, using a magnetic material containing silicon element in an amount of from 0.05 to 1.5% by weight based on the weight of iron element and having been treated with a silane coupling agent. The magnetic material used has an octahedral particle shape, the magnetic material has a smoothness of less than 0.30 according to experiments made by the present inventors, and also the magnetic material has portions not well made hydrophobic because the treatment of the magnetic material with the silane coupling agent is made by a wet process. Moreover, the magnetic material is used in an amount less than 70 parts by weight based on 100 parts by weight of polymerizable monomers and has a volume average particle diameter of 7.5 xcexcm. Accordingly, it is sought to provide a magnetic toner much more improved in developing performance and resolution.
In full-color copying machines or full-color printers, it has been common to use a method in which, using four photosensitive members and a beltlike transfer member, electrostatic latent images respectively formed on the photosensitive members are developed by the use of a cyan toner, a magenta toner, a yellow toner and a black toner, and the toner images thus formed are direct-pass transferred to a transfer-receiving medium being transported between the photosensitive members and the beltlike transfer member, followed by fixing of the toner images to form a full-color image, or a method in which a transfer-receiving medium is wound on the surface of a transfer member set opposingly to one photosensitive member, the transfer-receiving medium being wound by an electrostatic force or a mechanical action such as gripping, and the process of from development to transfer is carried out four times to obtain a full-color image.
In recent years, as transfer-receiving mediums for full-color copying or printing, it has become increasingly necessary to deal with various materials, e.g., not only sheets of paper usually used and films for overhead projectors (OHP) but also sheets of cardboard or small-sized sheets of paper such as cards and post cards. In the above method making use of four photosensitive members, the transfer-receiving medium is straight transported, and hence the method can be widely applied to various types of transfer-receiving mediums. Since, however, a plurality of toner images must be exactly superimposed on the transfer-receiving medium at its preset position, even a little difference in registration makes it difficult to form a high-quality image in a good reproducibility, and the transport mechanism for transfer-receiving mediums must be made complicated to cause problems of a low reliability and an increase in the number of parts. As for the method in which the transfer-receiving medium is attracted and wound on the surface of a transfer member, the transfer-receiving medium may cause a faulty close contact at its rear end because of a high stiffness of the transfer-receiving medium, consequently tending to cause faulty images due to faulty transfer undesirably. Similar faulty images tend to occur also in small-sized sheets of paper.
A full-color image forming apparatus making use of a drum type intermediate transfer member is proposed in U.S. Pat. No. 5,187,526 and Japanese Patent Application Laid-Open No. 4-16426. This U.S. Pat. No. 5,187,526 discloses that high-quality images can be formed when an intermediate transfer roller having a surface layer formed of polyurethane as a base has a volume resistivity below 109 xcexa9xc2x7cm and a transfer roller constituted of a similar surface layer has a volume resistivity of 1010 xcexa9xc2x7cm or above. In such a system, however, a high-voltage electric field is required in order to impart a sufficient transfer charge quantity to the toner when toners are transferred to the transfer-receiving medium. Hence, the surface layer constituted of polyurethane, in which a conductivity-providing material has been dispersed, may cause breakdown locally, so that image disorder tends to occur undesirably in the case of halftone images on which the toner is laid in a smaller quantity. In addition, application of such a high voltage tends to cause faulty transfer because of leakage of transfer electric currents as the transfer-receiving medium comes to have a lower resistance, in an environment of high humidity having a relative humidity above 60% RH. In an environment of high humidity having a relative humidity of 40% RH or below on the other hand, it may also tend to cause faulty transfer due to uneven resistance of the transfer-receiving medium.
Japanese Patent Application Laid-Open Nos. 59-15739 and 59-5046 disclose a relation between constitution making use of an intermediate transfer member and a toner. These publication, however, only discloses that a toner with an average particle diameter of 10 xcexcm or smaller is transferred in a good efficiency by means of a sticky intermediate transfer member. Usually, in the system employing an intermediate transfer member, the toner image must be once transferred from the photosensitive member to the intermediate transfer member and thereafter transferred from the intermediate transfer member to the transfer-receiving medium, where the transfer efficiency of toner must be made higher than the above conventional methods. In particular, when a full-color copying machine or full-color printer in which a plurality of toner images are transferred after development is used, the quantity of toner on the photosensitive member is larger than the case of monochromatic black toners used in black and white copying machines or printers and it is difficult to improve transfer efficiency when conventional toners are merely used. Also when usual toners are used, the melt-adhesion of toner or filming tends to occur on the surface of the photosensitive member or on the surface of the intermediate transfer member because of a shear force or frictional force acting between the photosensitive member or intermediate transfer member and the cleaning member and/or between the photosensitive member and the intermediate transfer member, so that the transfer efficiency tends to lower or, in the formation of a full-color image, the toner images corresponding to the four colors can not be uniformly transferred. Thus, problems tend to occur in respect of uneven colors and color balance.
Especially when the magnetic toner containing a magnetic powder is used as a black toner, the above problems tend to occur.
The various performances required for toners as stated above often conflict with one another, and yet in recent years it is more sought to satisfy all of them in a high performance. Under such circumstances, the magnetic powder that is a toner constituent material plays a great role and is sought to be made to have a higher function.
An object of the present invention is to provide a magnetic toner for developing electrostatic latent images that has solved the problems discussed above.
Another object of the present invention is to provide a magnetic toner for developing electrostatic latent images that can form high-quality images over a long period of time.
Still another object of the present invention is to provide a magnetic toner for developing electrostatic latent images that does not adversely affect the electrostatic latent image bearing member such as photosensitive member, the developer carrying member such as developing sleeve, and the intermediate transfer member.
A further object of the present invention is to provide a magnetic toner for developing electrostatic latent images that can form stable magnetic toner images in every environment.
A still further object of the present invention is to provide a production process that can preferably produce the above magnetic toner.
A still further object of the present invention is to provide an image forming method making use of the above magnetic toner.
To achieve the above objects, the present invention provides a magnetic toner for developing an electrostatic latent image, comprising magnetic toner particles containing at least a binder resin, a magnetic powder and a wax component, wherein;
(a) the magnetic powder;
1) has magnetic iron oxide particles the particle surfaces of which have been coat-treated with an organic surface modifying agent;
2) the magnetic iron oxide particles contain silicon element (Si) in an amount of from 0.4 to 2.0% by weight based on the weight of iron element (Fe); and
3) the magnetic iron oxide particles have an Fe/Si atomic ratio of from 1.0 to 4.0 at their outermost surfaces; and
(b) the magnetic toner particles have shape factors SF-1 and SF-2 as measured by an image analyzer, with a value of SF-1 of from 100 to 160, a value of SF-2 of from 100 to 140 and a value of (SF-2)/(SF-1) of not more than 1.0.
The present invention also provides a process for producing a magnetic toner containing magnetic toner particles, comprising the steps of;
i) mixing a polymerizable monomer, a magnetic powder, a wax component and a polymerization initiator to prepare a polymerizable monomer composition; wherein 1) the magnetic powder has magnetic iron oxide particles the particle surfaces of which have been coat-treated with an organic surface modifying agent, 2) the magnetic iron oxide particles contain silicon element (Si) in an amount of from 0.4 to 2.0% by weight based on the weight of iron element (Fe); and 3) the magnetic iron oxide particles have an Fe/Si atomic ratio of from 1.0 to 4.0 at their outermost surfaces;
ii) dispersing the polymerizable monomer composition in an aqueous medium to form particles of the polymerizable monomer composition;
iii) subjecting polymerizable monomers in the particles of the polymerizable monomer composition to polymerization in the aqueous medium to form magnetic toner particles; wherein the magnetic toner particles contain at least a binder resin, a magnetic powder and a wax component, and the magnetic toner particles have shape factors SF-1 and SF-2 as measured by an image analyzer, with a value of SF-1 of from 100 to 160, a value of SF-2 of from 100 to 140 and a value of (SF-2)/(SF-1) of not more than 1.0.
The present invention still also provides an image forming method comprising;
a charging step of externally applying a voltage to a charging member to electrostatically charging an electrostatic latent image bearing member;
a latent-image formation step of forming an electrostatic latent image on the electrostatic latent image bearing member thus charged;
a developing step of developing the electrostatic latent image by the use of a magnetic toner to form a toner image on the electrostatic latent image bearing member;
a first transfer step of transferring to an intermediate transfer member the toner image held on the electrostatic latent image bearing member;
a second transfer step of transferring to a transfer-receiving medium the toner image transferred onto the intermediate transfer member; and
a fixing step of heat-and-pressure fixing the toner image transferred onto the transfer-receiving medium;
wherein;
the magnetic toner comprises magnetic toner particles containing at least a binder resin, a magnetic powder and a wax component;
(a) the magnetic powder;
1) has magnetic iron oxide particles the particle surfaces of which have been coat-treated with an organic surface modifying agent;
2) the magnetic iron oxide particles contain silicon element (Si) in an amount of from 0.4 to 2.0% by weight based on the weight of iron element (Fe); and
3) the magnetic iron oxide particles have an Fe/Si atomic ratio of from 1.0 to 4.0 at their outermost surfaces; and
(b) the magnetic toner particles have shape factors SF-1 and SF-2 as measured by an image analyzer, with a value of SF-1 of from 100 to 160, a value of SF-2 of from 100 to 140 and a value of (SF-2)/(SF-1) of not more than 1.0.
The present invention further provides an image forming method comprising;
a charging step of externally applying a voltage to a charging member to electrostatically charging an electrostatic latent image bearing member;
a latent-image formation step of forming an electrostatic latent image on the electrostatic latent image bearing member thus charged;
a developing step of developing the electrostatic latent image by the use of a magnetic toner to form a toner image on the electrostatic latent image bearing member;
a transfer step of transferring to a transfer-receiving medium the toner image held on the electrostatic latent image bearing member; and
a fixing step of heat-and-pressure fixing the toner image transferred onto the transfer-receiving medium;
wherein;
the magnetic toner comprises magnetic toner particles containing at least a binder resin, a magnetic powder and a wax component;
(a) the magnetic powder;
1) has magnetic iron oxide particles the particle surfaces of which have been coat-treated with an organic surface modifying agent;
2) the magnetic iron oxide particles contain silicon element (Si) in an amount of from 0.4 to 2.0% by weight based on the weight of iron element (Fe); and
3) the magnetic iron oxide particles have an Fe/Si atomic ratio of from 1.0 to 4.0 at their outermost surfaces; and
(b) the magnetic toner particles have shape factors SF-1 and SF-2 as measured by an image analyzer, with a value of SF-1 of from 100 to 160, a value of SF-2 of from 100 to 140 and a value of (SF-2)/(SF-1) of not more than 1.0.